This invention relates to an improved method for using hollow fiber membranes to separate gas mixtures where the demand is not continuous.
The use of hollow fiber membranes to separate gas mixtures has been the object of considerable research during the past twenty years. U.S. Pat. No. 3,798,185 describes hollow fibers made from a variety of thermoplastics to which polyorganosiloxane has been added. U.S. Pat. No. 4,068,387 discloses a method for drying cellulose ester membranes for use in gas separation. A method for the recovery of hydrogen from ammonia synthesis purge streams utilizing membrane separation devices is described in U.S. Pat. No. 4,180,552. Multicomponent membranes for gas separations are claimed in U.S. Pat. No. 4,230,463. The separation of hydrogen sulfide from methane using capillary fibers is taught in U.S. Pat. No. 3,534,528.
The systems described in the prior art for the separation of a gas mixture via a semi-permeable membrane generally contemplate continuous operation. See, e.g., U.S. Pat. No. 4,119,417. U.S. Pat. No. 3,923,461 discloses the alternative routing of a carrier gas to either the permeate side of the membrane or to a detector, but intermittent operation is not explicitly discussed.
In many specific applications of membrane devices for gas separation, the separated gases are not required on a continuous basis. Since energy is expended in providing feed gas at operating pressure for separation, usually feed gas is only provided to the separation device when the separated gases are needed. On demand, the feed gas can rapidly be introduced to the separation device at operating pressure.
It has now been discovered that such pressure cycling can have a deleterious effect on the permeation characteristics of hollow fiber membranes. A method is disclosed for increasing the useful lifetime of hollow fiber membranes subject to intermittent use.